git-merge documentation: conflicting merge leaves higher stages in index

This hopefully concludes the latest updates that changes the
behaviour of the merge on an unsuccessful automerge.  Instead of
collapsing the conflicted path in the index to show HEAD, we
leave it unmerged, now that diff-files can compare working tree
files with higher stages.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-merge.txt

@@ -108,10 +108,12 @@ When there are conflicts, these things happen:
 2. Cleanly merged paths are updated both in the index file and
    in your working tree.
 
-3. For conflicting paths, the index file records the version
-   from `HEAD`. The working tree files have the result of
-   "merge" program; i.e. 3-way merge result with familiar
-   conflict markers `<<< === >>>`.
+3. For conflicting paths, the index file records up to three
+   versions; stage1 stores the version from the common ancestor,
+   stage2 from `HEAD`, and stage3 from the remote branch (you
+   can inspect the stages with `git-ls-files -u`).  The working
+   tree files have the result of "merge" program; i.e. 3-way
+   merge result with familiar conflict markers `<<< === >>>`.
 
 4. No other changes are done.  In particular, the local
    modifications you had before you started merge will stay the